1. Field of the Invention
This invention relates generally to the field of rocket propulsion engines. More specifically, the invention relates to rocket engines which provide thrust by discharging ionized particles in a selected direction to provide motive power for a vehicle.
2. Description of the Related Art
Engines which generate thrust by discharging ionized gas (plasma) from the engine are known in the art. U.S. Pat. No. 5,241,244 issued to Cirri, for example, describes one such engine. The engine disclosed in the Cirri ""244 patent generates plasma by imparting an electromagnetic field to neutral gas injected into an ionization chamber. Free electrons in the chamber are contained in the discharge chamber and are excited by imparting an electromagnetic field generated by a radio frequency generator operating at frequencies near the ionization resonance frequency. Excited electrons strike gas atoms inside the chamber, causing the gas atoms to be ionized. A grid at the exhaust end of the discharge chamber is electrically charged to attract the ions, causing them to leave the discharge chamber through the grid at high velocity. The discharging ions create the thrust exerted by the engine.
Another type of plasma discharge engine is known as a Hall effect plasma thruster. One type of Hall effect plasma thruster is described, for example, in U.S. Pat. No. 5,845,880 issued to Petrosov et al. The thruster in the Petrosov et al ""880 patent includes a chamber into which neutral gas such as xenon is injected. Electrons are emitted from a cathode proximate the discharge end of the thruster, and are accelerated toward the other end of the chamber by an anode onto which a high voltage is impressed. A magnetic field imposed on the chamber causes the electrons to move in a substantially helical path towards the anode, picking up speed as they travel. As the electrons come near the anode they collide with molecules of the injected gas, causing it to be ionized. Electrons tend to collect in a xe2x80x9ccloudxe2x80x9d near the exhaust end of the engine due to the magnetic field. Positively charged ionized gas atoms are electrostatically attracted to the electron cloud and exit the thruster at high speed. The magnetic field has much less effect on the path taken by the discharged ions because they are much more massive than electrons, and so the ions leave the thruster in a substantially straight path.
U.S. Pat. Nos. 4,815,279 and 4,893,470 issued to Chang describe another type of plasma discharge engine. The engine described in these patents includes an electrostatic plasma generator, such as a Marshall gun. Plasma from the generator is confined by a series of magnets and is directed to a discharge nozzle. The discharge nozzle is generally cone shaped. Contact between the plasma and the nozzle material is reduced by insulating the nozzle with neutral gas injected near the interior surface of the nozzle, and by focusing the plasma discharge using focusing magnets positioned near the nozzle inlet. The thrust developed by the engine described in these patents can be adjusted by varying the amount of neutral gas injected into the nozzle, thereby varying the mass flow through the nozzle which is directly related to the associated thrust.
One limitation of plasma discharge engines known in the art is that the specific impulse (thrust per unit mass of exhaust) of such engines cannot be easily controlled. Generally only the thrust can be directly controlled. For certain types of journeys, such as interplanetary travel, it would be desirable to have an engine which can operate at low thrust and very high specific impulse, so that high velocities, and perhaps even artificial gravity, can be developed during the journey. However, such engines would preferably have the capacity also to develop very high thrust when needed, such as during escape from planetary orbit, or reentry to orbit or planetary atmosphere.
Generally speaking, prior art plasma discharge type engines impart high discharge velocity to the plasma by imposing an electrostatic field to the plasma. The positively charged gas ions are caused to leave the engine at high velocity by being attracted to a negatively charged cathode, while the electrons remain behind. Necessarily, therefore, plasma discharge engines known in the art require an electrostatic neutralizer for the discharging ions so that electrostatic charge will not build up as a result of discharging only the positively charged ions from the engine.
An engine according to the invention comprises a controllable output plasma generator, a controllable heater for selectably raising the temperature of the plasma connected to an outlet of the plasma generator, and a nozzle connected to an outlet of the heater through which heated plasma is discharged to provide thrust. In one embodiment of the engine the plasma generator is a helicon generator. In one embodiment of the engine, the heater is an ion cyclotron resonator. In one embodiment of the engine, the nozzle comprises a radially diverging magnetic field disposed on the discharge end of the heater. A particular embodiment of the invention includes control circuits for selectably directing a selected portion of a total amount of available radio frequency power from an RF generator to the helicon generator, the remainder of the RF power going to the ion cyclotron resonator, so that the thrust output and the specific impulse of the engine can be selectably controlled.
In a particular embodiment of the engine, the plasma in the heater can be selectably recycled through the heater to further increase its temperature by including a selectably operable choke at the discharge end of the heater. In this embodiment, the choke consists of an axially polarized, variable amplitude magnetic mirror.
In still another embodiment of the engine, separation of the discharging plasma from the diverging magnetic field is improved by imparting an alternating magnetic field to the discharging plasma to xe2x80x9cstripxe2x80x9d it from the diverging magnetic field.
A method for propelling a vehicle according to the invention comprises generating a plasma, heating the plasma, and discharging the heated plasma through a nozzle. In one embodiment of the method, the generating is performed by a helicon generator. In one embodiment of the method, the heating is performed by an ion cyclotron resonator. In one embodiment of the method, the discharging is performed by exhausting the heated plasma through a radially diverging magnetic field disposed at one end of a chamber in which the heating takes place.
Another aspect of the invention is a method for adjusting an attitude of a vehicle, for example an outer space travelling vehicle. This aspect of the invention includes generating a plasma, heating the plasma, discharging the heated plasma through a nozzle, and directing a selected fraction of a total electrical power on the vehicle to the plasma generating, the remainder of the total electrical power being directed to the plasma heating. Selective power direction enables selectively varying a thrust and a specific impulse of propulsion. In one example, large and/or rapid attitude changes can be effected, where required, by selecting high thrust. For ultra precise pointing, where very precise attitude changes or attitude maintenance are required, high specific impulse can be selected.